Selective catalytic reduction (SCR) with ammonia as the reductant is often used for removing NOx from exhaust gases in automotive applications. Ammonia may directly be supplied from an ammonia-containing solid ammonia storage medium capable of reversibly ad- or absorbing and desorbing ammonia. The ammonia is consumed during driving whereby the storage medium is depleted. A device similar to a level sensor in a liquid tank is desirable to alert the driver that the ammonia storage tank will soon be empty in which case the car will start to pollute the environment with NOx.
Other applications of solid ammonia storage media include delivery of ammonia as fuel to for example fuel cells in energy generation applications. Here, a determination of filling level is of high value as well.